In general, a composite is a material that contains two or more components. Each component can contribute unique properties to the composite. As a result, the composite can have the advantageous properties of each component, all of which would not be present in a material that is lacking one of the components. Some composite materials can be particularly well-suited for use, for example, in optical, electronic, optoelectronic, magnetic, or catalytic applications.
In optical applications, the composite material can form a waveguide or an optical amplifier. Optical amplifiers utilize a gain medium to amplify optical radiation. In an amplifier, a source excites the gain medium to produce a population inversion between high and low energy states of the gain medium. The excited gain medium can amplify optical radiation at energies overlapping the energy differences between the high and low energy states of the population inversion because stimulated emission of radiation from the medium is more efficient than absorption of light. In general, a laser utilizes a cavity to supply feedback to an excited gain medium to cause amplified spontaneous emission. A laser cavity can include a series of optical components, such as mirrors, arranged relative to the gain medium to reflect radiation back into the cavity and thereby provide feedback. For example, a gain medium can be placed into a stable or unstable resonator. Alternatively, amplified spontaneous emission can occur in an excited gain medium without external optical components if the gain medium has a length, L, and gain coefficient, G (cm−1) sufficient to satisfy the expression:G·L>>1  
where the gain coefficient, G, is related to the stimulated emission cross section and the difference in the population densities of the high and low energy states generated by the population inversion.
Conventional solid-state and gas lasers and amplifiers generally provide very specific spectral outputs depending upon the laser material. If a spectral output other than that achievable with available gain materials or a less specific spectral output is desired, dye lasers or tunable optical parametric oscillators (OPO) or amplifiers (OPA) can be used. Dye lasers are large and bulky and also require fluid components that can be toxic.